J . Am. Chem. SOC.1991, 113, 2884-2890
2884
of previous experimental and theoretical works reported in the literature for other molecular hydrogen complexes. The comparison is satisfactory. Theoretically calculated binding energies15 of the molecular hydrogen ligand in several complexes range from 10 to 21 kcal/mol. Experimental data23give values between 7 and I O kcal/mol, probably being underestimated by the interference of agostic effects in the ML5 fragment. In any case, all these values are of the same order as those presented in this paper, which are 12.0 and 1 1 .O kcal/mol for the stable compounds 2 and 3-par, respectively.
Conclusions The present a b initio study on [Fe(PH3)4H(H2)]+systems has shown the peculiar effects of the hydride ligand on the bonding between the hydrogen molecule and the metal. The presence of a hydride trans to the hydrogen molecule increases the strength of the bonding of hydrogen to metal relative to the same position being occupied by a phosphine. If the hydride ligand is placed in the cis position, there is an important distortion in the MLIH fragment, although this distortion is independent of the orientation of the hydrogen molecule. However, the bonding of the hydrogen molecule to the metal depends strongly on this orientation. Actually, the bonding between the hydrogen molecule and the metal is unaffected by ligands oriented perpendicularly to the plane defined by the atoms of the metal and the hydrogen molecule. When suitably oriented, the hydride ligand in the cis position exhibits a strong attractive interaction, the hydrogen molecule (23) (a) Gonzalez, A. A.; Zhang, K.; Nolan, S.P.; L o p de la Vega, R.; Mukerjee, S. L.; Hoff, C. D. Organomefallics 1988, 7, 2429-2435. (b) Gonzalez, A. A.; Hoff, C. D.Inorg. Chem. 1989, 28, 4295-4297.
being polarized in a process which can be considered to have essentially an electrostatic origin. The optimized geometries present small distortions relative to a regular octahedron, but the coordination of the hydrogen molecule is very sensitive to such distortions. Bending of phosphines away from the hydrogen molecule increases the strength of its coordination and magnifies the different effects caused by the presence in a trans position of the hydride and phosphine ligands. This fact makes the optimization of the relative position of spectator ligands especially important in these cases. The quantitative value of the results obtained is indeed relative owing to simplifications, namely, (a) the modeling of the systems, (b) the lack of correlation energy, and (c) the possible decompensations in the employed basis set. However, our results agree qualitatively with available experimental data, so we think that they are quite useful as a tool in interpreting qualitatively these types of complexes. Understanding the peculiarities of the hydride ligand in molecular hydrogen complexes is a first step in the comprehension of the chemical interaction between these two ligands, Le., the interchange reaction between hydrogen atoms. Really, the interaction between the hydride and the hydrogen molecule observed for the ~ i s - [ F e ( p H ~ ) ~ H ( Hcomplex ~ ) l + is directly related to these processes. The interchange reaction, which has an obvious chemical interest, is presently the subject of theoretical research in our group.
Acknowledgment. We wish to thank Professor 0. Eisenstein from Orsay for helpful discussions. This work received financial support from the Spanish ‘DirecciBn General de InvestigaciBn Cientifica y TEcnica” under Project No. PB86-0529.
Properties of Small Group IIIA Hydrides Including the Cyclic and Pentacoordinate Structures of Trialane (A13H9)and Trigallane (Ga,Hg): Can Dialane Be Isolated? Brian J. Duke,+Congxin Liang, and Henry F. Schaefer III* Contribution from the Center for Computational Quantum Chemistry, University of Georgia, Athens, Georgia 30602. Received July 19, 1990
Abstract: The structure of small group IIIA hydrides up to the trimers are studied using the self-consistentfield (SCF), single and double excitation configuration interaction (CISD), and single and double excitation coupled cluster (CCSD) methods in conjunction with a double-zeta plus polarization basis set (DZP). The dimerization energy for the unknown dialane is found to be significantly greater than that for the recently synthesized digallane. Unlike the analogous boron compound, cyclic trialane(9) and trigallane(9) incorporate planar six-membered rings according to the DZP SCF predictions. Acyclic trimer structures with pentacoordinated heavy atoms are also considered. The pentacoordinate trialane(9) is nearly isoenergeticwith the cyclic structure,and the pentacoordinate triborane(9) is only slightly higher in energy than the cyclic triborane. This new pentacoordinated triborane may be involved in the pyrolysis of diborane and may be the key to the resolution of a dispute concerning the kinetics of this process.
1. Introduction
With the recent successful synthesis of digallane,’ having a diborane-like structure, the absence of dialane poses a challenge to both experimentalists and theorist^.^^^ Two hypotheses can be made. First, dialane may be sufficiently stable to exist,3 but no suitable synthetic route has yet been discovered. Certainly the key to the synthesis of digallane, and also galloborane: was Downs’ use of the monochlorogallane dimer as a precursor. The stability of dialane, as compared to diborane and digallane, has been studied ‘Permanant address: School of Chemistry and Earth Sciences, Northern Territory University, P.O.Box 40146, Casuarina, N T 081 I , Australia.
0002-7863/91/1513-2884$02.50/0
by Lammertsma and Leszczynski3who reported the dimerization energies for borane, alane, and gallane obtained from molecular electronic structure theory with inclusion of electron correlation effects by Mdler-Plesset perturbation theory. They concluded that dialane should be experimentally observable since its binding ( I ) Downs, A. J.; Goode, M. J.; Pulham, C. R. J . Am. Chem. SOC.1989, 111, 1936.
(2) Liang, C.; Davy, R. D.; Schaefer, H. F. Chem. Phys. Lett. 1989, 159, 393. (3) Lammertsma, K.; Leszczynski, J. J . Phys. Chem. 1990, 94, 2806. (4) Pulham, C. R.; Brain, P. T.; Downs, A. J.; Rankin, D.W. H.; Robertson, H. E. J . Chem. Soc., Chem. Commun. 1990, 177.
0 1991 American Chemical Society
Properties of Small Group IIIA Hydrides
J . Am. Chem. SOC.,Vol. 113, No. 8, 1991 2885
Table 1. Total Energies (in au) of Borane, Alane, and Gallane and Their Dimers Obtained Using the DZP Basis Set method SCF"
~~
BH3 B2H6 AlH3 -52.81686 -243.623 -26.392 18 -53.050 84 -243.718 -26.502 43 CISD~ -26.507 74 -53.071 72 -243.723 Davidsonb -243.721 CCSDb -26.505 60 -53.069 33 'At S C F optimized geometries. At CISD optimized geometries.
energy is higher than that for digallane and is just slightly lower than that for diborane. However, the basis sets and theoretical methods employed for the three systems were not directly comparable. In addition, they did not consider the possibility that the association of alane units to give polymers larger than the dimer (and ultimately the known infinite solid structure) may be energetically more favorable than for borane and gallane. One piece of experimental evidence for this possibility is that while dimethylgallane prefers a dimer to the trimer (I), the opposite is true for dimethylalane.s Thus, the present investigation is designed to determine the energetics for formation of dimers and trimers of borane, alane, and gallane to test the above hypotheses. H
I
No trimers of borane, alane, or gallane have been characterized. Triborane(9) (B3H9)has been assumed to be an intermediate in the pyrolysis of diborane to give higher boranes such as tetraborane( IO), pentaborane(9), and pentaborane( 1 1).6 It has also been reported that triborane was observed by mass spectrometry during the association process of borane and diborane,' but its structure is not known. Fehlne? suggested a structure with a single bridge formed from a terminal H of B2H, to BH, (11). This structure can rearrange, explaining H-D exchange data in the pyrolysis reaction. However, a recent a b initio study9 on the early stage of the pyrolysis of diborane focused only on the cyclic on this structure (1) of B3H9. A number of a b initio compound concluded that it has C,, symmetry with the bridging hydrogens about 0.46 %r. above the plane of the boron atoms.I4 There are, however, a few substituted trimers experimentally known. DimethylalanelS and dimethylgallane trimersSare thought to have the cyclic structure (I). Interestingly, mixed trimers, methylaluminum bis( tetrahydr~borate)'~-'' and hydridogallium bis(tetrahydrob0rate) (111),18*'9 have structures with penta(5) Baxter, P. L.; Downs, A. J.; Goode, M. J.; Rankin, D. W. H.; Robertson, H. E. J. Cfiem. Soc., Cfiem. Commun. 1986, 805. (6) Greenwood, N. N.; Greatrex, R. Pure Appl. Cfiem. 1987, 59, 857. (7) Fridmann. S.A.; Fehlner, T. P. J. Am. Cfiem. Soc. 1971, 93, 2824; Inorg. Cfiem. 1972, I I , 936. (8) Fehlner, T. P. Boron Hydride Cfiemisfry;Muetterties, E. L., Ed.; Academic Press: New York, 1975; p 175. (9) Stanton, J. F.; Lipscomb, W. N.; Bartlett, R. J. J. Am. Chem. SOC. 1989, 1 1 1 , 5165. (IO) Ortiz, J. V.; Lipscomb, W. N. Cfiem. Pfiys. Lett. 1983, 103, 59. ( I I ) Pepperberg, I. M.; Halgren, T. A.; Lipscomb, W. N. Inorg. Cfiem. 1977, 16, 363. (12) McKee, M. L.; Lipscomb, W. N. Inorg. Cfiem. 1982, 21, 2846. (13) McKee. M. L.; Lipscomb, W. N. Inorg. Chem. 1985, 24, 2317. (14) Stanton, J. F.;Lipscomb, W.N.; Bartlett, R. J.; McKee, M. L. Inorg. Cfiem. 1989, 28, 109. (IS) Wartik, T.; Schlesinger, H. I. J. Am. Cfiem. SOC.1953, 75, 835. (16) Oddy, P. R.; Wallbridge, M. G. H. J. Chem. SOC.,Dalton Trans. 1976, 869. (17) Barlow, M. T.; Dain, C. J.; Downs, A. J.; Thomas, P.D. P.; Rankin, D. W. H. J. Cfiem. SOC.,Dalton Trans. 1980, 1374. (18) Downs, A. J.; Thomas, P. D. P. J. Cfiem.Soc.,Cfiem.Commun. 1976, 826. (19) Barlow, M. T.; Dain, C. J.; Downs, A. J.; Laurenson, G.S.; Rankin, D. W. H. J. Chem. SOC..Dalton Trans. 1982, 597.
4% 12 80 63 77
-487.291 64 -487.48449 -487.501 74 -487.500 27
GaH, -1924.8 18 06 -1924.913 96 -1924.91924 -1924.917 13
GaZH6 -3849.662 15 -3849.855 8 I -3849.874 5 1 -3849.872 84
coordinated metals, although the cyclic structure is topologically favored for B3H9.20 No theoretical studies have been carried out to date for this type of structure. Therefore, this study is designed to investigate (1) the dimerization of borane, alane, and gallane at high, uniform theoretical levels and (2) the structures and energetics of trimers of borane, alane, and gallane; in this regard, structures I, 11, and I11 will be considered.
2. Theoretical Methods In order to obtain comparable results for compounds containing all three group IIIA elements, comparable basis sets must be selected for all atoms. As a compromise between economy and accuracy, double-zeta plus polarization (DZP) basis sets were selected. This is probably as large a basis as can be readily used for high-level studies of the largest molecule, trigailane. In addition, this basis set makes possible geometry optimizations of the dimers using method including electron correlation. Thus, the Huzinagaz'-Dunningz2 standard double-zeta basis sets for boron (9s5p/4s2p), aluminum (1 ls7p/6s4p), and hydrogen (4s/2s) were used with the addition of polarization functions [ad@) = 0.70; ad(A1) = 0.40; ap(H) = 0.751. For gallium, we first studied the 10s8p2d (TZP) contraction of Dunning's 14sl lp5d primitive basisz3used by Liang et aL2 for digallane (a set of d polarization functions, ad = 0.16, was added). It is found that adding 2slp diffuse functionsU to that basis set introduces essentially no change on the geometry of digallane. Decontracting the d functions to 3d also has negligible effect. We then further contracted Liang et al.'s basis set to 7s5p2d (designated as DZP) and found that it gives nearly comparable results to the T Z P contraction: the DZP predicted bond lengths are shorter by less than 0.03 A and bond angles larger by
